Cancer Constellation

Cancer the Crab is the faintest constellation of the zodiac, a quiet, easily overlooked patch of sky between the twins of Gemini and the lion of Leo. In Greek myth it commemorates a small crab that pinched at the heel of Hercules during his battle with the multi-headed Hydra, and was crushed for its trouble — but was honored with a place among the stars regardless by the goddess Hera. What Cancer lacks in bright stars, it makes up for with one of the loveliest naked-eye star clusters in the sky, the Beehive, glowing at its heart.

Cancer at a Glance

• AbbreviationCnc
• Genitive FormCancri
• MeaningThe Crab
• Constellation FamilyZodiac
• Area506 sq. degrees (31st largest of 88)
• Right Ascension7h 55m – 9h 23m
• Declination+6.5° to +33.1°
• Brightest StarAl Tarf (magnitude 3.53)
• Best Evening ViewingMarch, visible 90°N to 60°S
• Sun's Actual TransitJuly 20 – August 10 (astrology: June 21 – July 22)

Mythology and History

According to the myth, Hera, who resented Hercules as the product of one of Zeus's affairs, sent the crab to distract him during his second labor, the fight against the Lernaean Hydra. The crab did little damage before Hercules crushed it underfoot, but Hera rewarded its loyalty by placing it among the stars. Because its role in the myth was so minor, Cancer was never given bright stars to match its more famous zodiac neighbors — ancient astronomers still recorded it faithfully, and it was one of the 48 constellations catalogued by Ptolemy in the 2nd century AD. In classical antiquity, Cancer also marked the point where the Sun reached its northernmost declination at the summer solstice, which is why the Tropic of Cancer on Earth's map still carries its name, even though precession has since shifted that solstice point into Gemini.

Notable Stars in Cancer

Cancer's dimness is part of its character: its brightest star, Al Tarf (Beta Cancri), only reaches magnitude 3.53, an orange giant roughly 290 light-years from Earth. Acubens (Alpha Cancri), a white star marking one of the crab's claws, is fainter still. Two other named stars, Asellus Borealis and Asellus Australis — the "Northern" and "Southern Donkeys" — flank the Beehive Cluster and were associated in ancient skylore with the animals said to have carried Dionysus into battle.

Deep-Sky Objects

The reason to visit Cancer is the Beehive Cluster (M44), also called Praesepe or "the Manger," one of the nearest open star clusters to our solar system at roughly 577 light-years. Under a dark sky it is visible to the naked eye as a soft glow, and binoculars resolve it into dozens of individual stars. Cancer also contains M67, a much older and more tightly bound cluster that astronomers prize for studying how stars of similar age and origin evolve differently over billions of years.

• Beehive Cluster (M44, Praesepe) — one of the closest open star clusters to Earth, a hazy swarm visible to the naked eye and resolved into dozens of stars through binoculars
• M67 — a much older open cluster, among the most-studied in the sky for the ages and compositions of its roughly 500 member stars

Neighboring Constellations and How to Find It

Cancer sits between the brighter constellations Gemini and Leo, and because it has no star brighter than magnitude 3.5, it's easiest to find by first locating the Beehive Cluster with binoculars, roughly midway between Pollux (in Gemini) and Regulus (in Leo). It climbs highest in the evening sky around March, and needs a reasonably dark, moonless night to appreciate fully. Cancer borders Lynx, Gemini, Canis Minor, Hydra, Leo, and Leo Minor.

Cancer is also associated with the Delta Cancrids meteor shower, visible to observers who track where these annual displays radiate from.